The invention concerns a method and equipment for bidirectional data transfer, whereby the transmission is implemented using a protocol between devices that are partners in communication ("communicators") having both a transmitter and a receiver, and whereby the information is transmitted in serial bit form and the direction of the data streams is switchable.
By means of such a method, data transmission can be implemented between communicators - such as computers and printers - or other intelligent electronic systems which are not connected by cable, with infra-red light or radio used as the transmission medium. Methods such as this can also be used in connection with communication networks, whereby information can be exchanged between communicators which are connected by cable within a network and those which are not connected to the network by cable.
For communication in networks with several participants, the network must be self-administrating, i.e. it must be established which participants can communicate with each other at which points in time and under which criteria. The entire set of rules and conventions for network administration and the automatic implementation of an orderly information exchange between communicators make up the protocol. For complex network administration the protocol is based on a decentralised administration, in which every participant can be "Master" and undertakes part of the administration task. It is established with the protocol how a participant is to use and then release the network.
Known protocols for such networks are the CSMA/CD protocol (carrier sense, multiple access/collision detection, e.g. Ethernet) and the CSMA/CA protocol (carrier sense, multiple access/collision avoidance). The principle difference lies in that for the CSMA/CD protocol, during conflicts, each communicator suspends its transmission, and only begins a new attempt to access the network (bus) after a certain wait time. In this way, a period of waiting results, during which no exchange of information takes place. For CSMA/CA protocols on the other hand there is no such wait time. The network loading is therefore less efficient for the CSMA/CD protocol compared to the CSMA/CA protocol.
For any CSMA protocol to be realised the following conditions must be fulfilled:
Master capability: each participant must be able to take control of the transmission medium independently, based on its transmission requirements. PA1 Listening ability: each participant must be able to observe the traffic on the network and use this as a criterion for the take-over of the network. A characteristic of a network with electrical connections and with a bus structure is the fact that all bus participants can listen to each data on the bus quasi-simultaneously (e.g. through "wired-or" output stages). PA1 Ability to interrupt: each participant must be able to establish whether, during use of the network, another participant is similarly using the network, so that in such a case it can immediately cease its transmission. PA1 Bidirectionality: the transmission medium must be capable of transmitting information bidirectionally. PA1 Simultaneousness: transmission medium and communicators must be arranged in such a way that the simultaneous use of the transmission channel by several communicators is possible. This means, amongst other things, that for an electrically connected network with a bus structure, the use of push-pull output stages is not possible. PA1 increase in the actual Baud rate through the transmission of two bits in the original signal element; PA1 increase in the redundancy of the data transmission through double bit representation; PA1 introduction of a direct service channel, e.g. for the transmission of the control signals "Clear to send" (CTS) and "Request to send" (RTS) with RS 232 links and modem applications; PA1 reduction in the energy requirements through the omission of transmission of the logic "O" value.
The above conditions imply that the participants must remain able to receive during transmission. For this reason CSMA/CA protocols have until now been used only in base-band transmission techniques.
The use of CSMA protocols is therefore not possible if, because of the transmission medium or method, a communicator cannot simultaneously listen to what other communicators are sending whilst itself transmitting data. Radio channels are an example of this, whereby other stations cannot be heard during transmission because of the extreme variations in field strengths. Another example are transmission systems based on fibre optics, in which the information is only transmitted unidirectionally because of the space multiplexing method used.
Another known method, in which data are transmitted using infrared light, uses a half-duplex method, whereby the data is sent in the form of data packets. Here also, a CSMA protocol cannot be used, as the aforementioned condition of simultaneous listening and sending is not fulfilled. With the abandonment of the use of such a protocol, the efficiency of the data transmission, and the time behaviour, deteriorate, in that (for example) long wait times result between a transmit requirement arising and the start of the actual data transmission.